This invention relates to heat-resistant electrically insulated wires for use as windings and wirings in electric equipment such as motors and electromagnets, and a method for preparing the same.
Recently, electrically insulated wires in the form of a conductor having a heat resistant ceramic coating thereon have been often used in proximity of the core of a nuclear reactor or in a high temperature atmosphere. Since ceramics, however, are generally very hard and fragile, wires having a ceramic coating have a substantially poor flexibility. Such ceramic-coated wires are difficult to carry out mechanical working or fabrication, for example, by bending, and are only applied to limited areas. Cracks often occur in the ceramic insulating coating during handling because of the lack of flexibility, and the ceramic insulating coating tends to peel because of insufficient adhesion between the ceramic coating and the metallic conductor. Such cracked or peeled coatings cannot ensure the satisfactory insulation of wires.
Japanese Patent Application Publication No. 48-2396 (Y. Matsuda et al., 1973) discloses a method for preparing a ceramic insulated wire in which a wire having a green insulating coating layer which has not been fired into a ceramic form or a semi-finished wire is subjected to mechanical working, for example, coil winding before it is fired at elevated temperatures to convert the coating layer into a ceramic layer. A similar method is disclosed in Eugene Cohn et al., U.S. Pat. No. 3,352,009. The coating layer which is to be fired after working may be prepared by either of the following methods:
(1) Onto a conductor is applied a mixture consisting of vitreous fine powder, a binding resin for imparting flexibility to the resultant coating, and a suitable solvent (so-called "enamel frit"). PA1 (2) Onto a conductor is applied a mixture consisting of vitreous fine powder, clay and water (so-called "enamel slip"). The resulting coating is then impregnated with a binding resin for imparting flexibility thereto.
In these prior art methods, the resin used as a binder must be completely eliminated in the subsequent firing step. For this reason, the preferred binder is a resin which tends to be readily decomposed and eliminated at relatively low temperature, for example, methacrylic ester resins. Accordingly, the material to be converted into a ceramic form should be a frit which can be sintered or softened and fused at a relatively low temperature approximate to the decomposition temperature of resins. Since such a frit usually contains a substantial amount of alkali metals such as sodium and potassium, the ceramic coating obtained by firing the frit has some drawbacks as poor electrical characteristics at elevated temperatures and low resistance to thermal shock.
To solve the above-mentioned problems, the inventors have attempted to prepare a ceramic insulated wire using as a binder a silicon resin which has a higher decomposition temperature than the prior art resins and is decomposed into a residue capable of binding ceramic particles. The mixture contains inorganic fine powder having improved good electrical insulating properties at elevated temperatures, such as high melting crystal particles and glassy particles, a silicon resin, and a diluent. The mixture is applied onto a conductor and then heated to the curing temperature of the silicon resin, thereby curing the resin. Mechanical working such as coil winding is carried out at this point. Thereafter, the formed wire is heated to an elevated temperature for decomposing the silicon resin to cause the organic contents to disappear and render the coating ceramic, thereby forming on the conductor a ceramic layer entirely and firmly bonded to the conductor. Since this method uses a silicon resin having a high decomposition temperature as a binder for imparting flexibility, the inorganic powder to be converted into a ceramic form may have a high melting or softening point. Accordingly, glassy fine powders which contain only a trace amount of alkali metals such as sodium and potassium may be employed. This ensures the provision of a ceramic insulated wire which has improved electrical properties at elevated temperatures and improved thermal shock resistance as compared with prior art ceramic insulated wires. In addition, the silicon resin allows the resultant ceramic layer to be firmly bonded to the conductor since the residual material resulting from the decomposition of silicon serves as a binder of inorganic powder particles. It is also possible to use inorganic fine powders having an extremely high melting point.
It should be noted that in some types of electrical equipment or in certain operating conditions thereof, heat-resistant electrically insulated wires are not subjected to such a high temperature as requiring insulating ceramic coatings during normal operation, but only during abnormal operation. There is a strong demand for developing a heat-resistant insulated wire adapted for use in such conditions. Continuing researches, the inventors have succeeded in developing a novel heat-resistant electrically insulated wire capable of meeting the abovedescribed requirements.